The electronically controlled fuel injection system has conventionally controlled an amount of fuel to be supplied to an engine by controlling valve opening time of a fuel injection valve.
In such a conventional fuel injection system, it is impossible to inject a constant amount of fuel in accordance with the valve opening time, unless a differential pressure is kept to be constant between the pressure of fuel to be supplied to the fuel injection valve and the intake pressure in the vicinity of an injection hole of the fuel injection valve.
Accordingly, there has conventionally been provided a pressure regulator for controlling the pressure of fuel to be supplied to a fuel injection valve from a fuel pump, and to a reference pressure chamber of the pressure regulator has been introduced the inlet negative pressure from the downstream of a throttle valve. If a differential pressure between the pressure in the reference pressure chamber and the pressure of fuel supplied from the fuel pump exceeds a predetermined value, a return passage is forced to open for returning fuel to the fuel pump to thereby keep the differential pressure to be constant (See Japanese Unexamined Patent Publication No. 60-212634).
In such an apparatus as above mentioned in which the pressure of fuel to be supplied to a fuel injection valve is controlled by adjusting an amount of fuel which is returned by the pressure regulator, the fuel pump is controlled to discharge a slight larger amount of fuel than required for maintaining the aforementioned differential pressure to be constant even if an amount of fuel required by an engine increase. Therefore, a slight amount of fuel required by an engine has resulted that much of fuel is returned back to a fuel tank as return fuel.
The return fuel brought back to the fuel tank from the pressure regulator is warmed up by heat generated by the engine. Accordingly, the temperature of fuel contained in the fuel tank is increased by the warmed return fuel, and hence there are generated fuel vapors in the fuel tank. Thus, it has been desired to reduce an amount of return fuel as much as possible.
Thus, it has been suggested to control discharge of a fuel pump in accordance with an amount of fuel required by an engine. However, in this suggestion, it has been necessary to control the fuel pump so that the pump discharges a little larger amount of fuel than required, so as to disregard the dispersion and variation in discharge of a fuel pump, with the result of generation of return fuel.
Another system has been proposed for avoiding the generation of return fuel. In this system , there is provided a sensor for detecting the pressure of fuel present in a fuel supply passage, and the discharge of a fuel pump is controlled in accordance with the fuel pressure detected by the sensor.
However, in such a system as above mentioned in which the detected fuel pressure is fed back to control discharge of a fuel pump, for instance, during transition period in which a required fuel amount is incrementally changed, the discharge of the fuel pump is controlled to increase after the fuel pressure has been decreased as the required fuel amount is incrementally changed. This brings the result that the fuel pressure decreases due to response delay in control and also due to supply delay of fuel, and hence an amount of fuel required by an engine may not be supplied quite precisely during transient driving.
The invention aims to overcome the aforementioned problems, and thus it is an object of the present invention to provide a system in which it is possible to prevent generation of return fuel causing the temperature rise in a fuel tank, and also possible to securely force-feed a required amount of fuel even if discharge of a fuel pump is varied due to the variation and/or degradation of the fuel pump.
Another object of the present invention is to make it possible to accomplish a correction control with high accuracy in order to compensate for the variation in discharge of the fuel pump generated due to the variation and/or degradation of the fuel pump.
A further object of the present invention is to make it possible to secure the follow up to the fuel pressure during transient driving, while preventing generation of the return fuel.